Adaptive Theme Selection Based on the Terminal Color Scheme

[luca-schlecker]

Fixes #8899, #10281

Overview

Introduce theme = { light = "<name>", dark = "<name>" } syntax inside config.toml to allow adaptive theme selection based on the terminal color scheme.

Theme selection is based on terminal-colorsaurus which has also been used in delta and bat.

Considerations

• If the theme detection fails, dark mode is chosen by default.
• The new syntax using an untagged enum ensures existing configs remain valid.
• Theme validation (ensuring the theme name exists) is only done for the detected theme. This would defer errors if a missing theme is specified but reduces code changes.
• terminal-colorsaurus' code is only called in case an adaptive theme has been specified, guaranteeing practically zero overhead for static themes.

[cmoog]

There seems to be a small bug when this runs via :config-reload

[luca-schlecker]

This bug turns out to be a bit trickier. I've spent a few hours and couldn't come up with a clean solution. Crossterm and Colorsaurus are interfering with each other here because Colorsaurus, instead of querying the operating system, uses ANSI escape codes to get the terminal background color and determines the active color scheme that way. This approach has some benefits like allowing theme detection through an ssh session.

I believe the interference is happening as soon as the future from the async function input_stream.next() (Application::event_loop_until_idle) is still pending whilst trying to detect the color scheme.

I've pondered a bit and could think of the following workarounds:

• Use dark-light to query the operating system instead (thus no terminal i/o required, but less flexible).
• Use a Semaphore to prevent input_stream.next() from being called when trying to detect the color scheme. For this to be effective the Semaphore would have to be locked before this tokio::select!, increasing latency of config reloads as the select-call would have to finish first.
• Tie theme detection directly to input events (Application::event_loop_until_idle):

Some(event) = input_stream.next() => {
    // <-- theme detection works here without interference
    self.handle_terminal_events(event).await;
    // <-- also here
}

• Only determine the terminal color scheme once at startup and don't update it on config reload.

Let me know what your thoughts on this are.